I'm guessing it's the poison arrow tree-frogs of South and Central America. However, their poison is secreted through the skin, so if you're using the definition which says that venom is "a toxin delivered by a bite or a sting" I'd have to go with one of the species of box jellyfish floating around Australia. But I am not a toxicologist, so take it with a grain of salt.

Other remarkably venomous creatures include the inland taipan, the cone snail, the stonefish, the funnel-web spider, various sea snakes, the blue-ringed octopus (which gets my vote as the cutest deadly animal out there), and certain salamanders.

The poison arrow frog and certain salamanders would have to top most people's list of "The World's Most Poisonous Animals." Just two micrograms of toxin from the poison arrow frog is enough to kill a human. A "microgram" is a very small amount. The ink in the period at the end of this sentence will weigh around six micrograms.

Venomous animal:

The prize for "The World's Most Venomous Snake" goes to the inland taipan of Australia. But the top prize, "The World Most Venomous Animal," should go to the box jellyfish, which is found in the waters around Asia and Australia. They have long tentacles with stings at the very ends.

Hook-Nosed Seasnake is a snake which has the most lethal venom tested so far. Only 1.5 milligrams of its venom will kill a human being. Well, that is for snake, but about which animal that has the most potent venom, then I don't know.

Schröder is right. The most venomous animal on the Earth is Chironex fleckeri, a huge squared jellyfish with sixty tentacles of 3 m long each one known as sea-wasp. This jellyfish has many blisters (20 000 aproximately) on its surface which contain between 1.5 and 4 ml of poison. The poison is a neurotoxin (its LD50 is about 20 mg/kg so with only 1.4 mg of poison, this animal could kill an adult person) which affects the nervous system (producing ulcerations and necrosis) very quickly and then occurs a heart failure.